[HOT] Broken (multi)polygon cleanup

Michael HeiƟmeier michael63 at digital-filestore.de
Sat Mar 18 11:54:17 UTC 2017

Hi all,

Jochen Topf, 2017-03-18 11:20:
> On Fri, Mar 17, 2017 at 10:48:00AM -0400, john whelan wrote:
>> I've read it through but remember like many mappers I do not have a Ph.D.
>> in GIS, just a degree in Chemistry so I'm usually not too bad on logic.
>> The first thing that stands out is the message is too complex for the
>> intended audience.
> Yes, and no. The documentation we have now is trying to be complete and
> thorough. The intended audience are, first, power mappers. I am totally
> aware that it is too much for the average mapper, but that's why I need
> everybodies help not only the fix things, but also to translate the
> problem into words that the different audiences can understand.
> I know that HOT has capable mappers that understand OSM well and have
> written documentation for new HOT mappers to use. I hope I can reach
> those people here so they can help.
>> The second "The *new style* tagging option is the recommended tagging
>> option these days, but some mappers still disagree."  So we don't have
>> complete agreement, fine.
> Well, that was me being overcautious. I haven't heard any disagreements
> in the last months. So I don't think there is any major disagreements
> left. I have removed that sentence from the documentation.
>> The third is the main problem areas seem to be some rendering systems
>> prefer one method over the other.  "Are we mapping for a specific rendering
>> system now?"
> No, we are not. I am not sure why you got that impression from the
> documentation. Can you point me to a specific sentence? I have
> highlighted Osm2pgsql, because it is by far the most often used basis
> for rendering maps and it is what powers the main OSM map. So changes
> there will affect more people. And it also means that basically every
> other system is trying to mimic what Osm2pgsql is doing (right or
> wrong), because they have to keep "in sync" with the main map that OSM
> mappers use to verify their work.
> Currently all rendering systems have to handle old- and new-style
> multipolygons and in the future they wont have to do that any more. This
> makes all systems simpler.
>> I suspect if we concentrate on unclosed ways and duplicate segments then
>> this is something that can be done without a Ph.D. in GIS concepts and
>> there is no disagreement.  Leave the more complex problem solving to others.
> I totally agree. That's why I have been splitting up the work into small
> Maproulette tasks, each with a very easy to understand and concrete
> description of what the fixing task is. I don't expect most mappers to
> be able to solve complex multipolygon cases. But there are plenty of
> simple cases around.

I gave the Maproulette tasks a try yesterday evening and found them easy to 
tackle. The advantage here is that they are typically localized to a small area, 
you do not have to take lots of data into account.

For the bigger picture, OSM Inspector is the tool of choice but I agree that 
many errors reported here are not easy to fix.
I will try to get an overview of errors made by relatively new contributors and 
see how we can improve documentation and learning materials. Errors made are not 
always what you think of when writing documentation, cf. this "single" building 
which contains self-intersections:

Not to forget the biggest multipolygon of all (although not technically 
organized as one), the coastline, which is more often broken than not.

>> I would recommend if you wish to use the resources of HOT that you talk
>> nicely to the HOT training group and see if they can sort out the message
>> and what training needs to be given to support these efforts including what
>> you would like mappers to avoid when mapping.
> I am hoping these people are listening here. (Are you?) Or do I have to
> go somewhere else to talk to them?

Yes, people active in the Training Working Group are listening here as well. We 
are aware that new mappers need guidance and made substantial improvements over 
the past months. Out chair, Nick Allen, mentioned this last month in his e-mail 
to the hot list 
(http://lists.openstreetmap.org/pipermail/hot/2017-February/012977.html). We are 
always open to suggestions for further material to be included there.

Best Regards


/PS: John,
> I have yet to see a HOT project that asks mappers to map country outlines.
I agree that these are rare but they do exist (http://tasks.hotosm.org/project/1407)

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